Conventionally, as a method of converging a light beam in light scanning, a method is employed which performs scanning conversion at constant speed on a plane to be scanned using an fθ lens or an arcsine lens and forms an image on the plane to be scanned.
Conventionally, a light scanning unit that scans a light beam emitted from a light source on the plane to be scanned by a light deflecting device is widely used in a writing system such as a laser printer and a reading system such as a barcode reader.
In the writing system that is required to be high definition, the light beam needs to converge to a beam diameter of about 50 μm, and thus a high-precision light scanning unit is required.
JP 2008-197336 A discloses a light scanning unit that reflects a light beam emitted from a light source by a light deflecting device composed of an oscillating mirror supported on a twisting brace, controls light projection timing of the light beam so that the scanning speed by the oscillating mirror becomes constant on the plane to be scanned, and converges the light beam on the plane to be scanned by a varifocal lens provided on the upstream side of the light deflecting device.
Further, JP 2012-141265 A discloses an apparatus that measure the distance to a plane to be scanned (object) by counting time from emission of a light beam to return back of reflected light in the case of two-dimensional scanning.